Update di-usage.texy

Correct several typos in code examples and also in text (formulation, typos etc.)

Done changes?
   - add missing semicolons
   - add namespace declaration statements
   - add caution block under "Other possibilities" title
   - correct wrong access modifiers on properties/methods
   - correct few typos in text
@dg But it is clearer to look at the cascade, isn't it?